Wireless power supplies in devices can be realized by means of inductive- and/or capacitive proximity coupling. Thereby, a transmitter unit generates an alternating electromagnetic field. This electromagnetic alternating field is coupled via coupled coils (inductive coupling) or an open capacitor (capacitive coupling) to the power sink, hereinafter referred to as a receiver unit.
With increasing distance between the transmitter unit and the receiver unit the coupling (k) decreases and reduces the receivable power at the receiver unit. Thereby, the coupling capacitance decreases where an open capacitor is used as being the coupling element, while in the case of coupled coils the leakage inductance increases. It is known in the art to compensate this effect by compensating the leakage inductance by capacitors, respectively by compensating the coupling capacitor with inductances. This creates at least one resonant circuit in the transmitter- and receiver units within the wireless power transmission link. These resonant circuits compensate for the leakage inductance and coupling capacitance, if the resonant circuits are tuned to the same resonant frequency and the wireless power transmission link operates at this resonant frequency. A nearly perfect compensation, using the secondary quality factor Qsec, is reached under the condition k·Qsec=1 (critical coupling). This corresponds to a wired connection. For values k·Qsec>1 several resonance frequencies occur, which, however, increase the frequency bandwidth but do not increase the amount of transmitted power of the wireless power transmission link. For this reason, a wireless energy- or power transmission link should operate as close as possible in the critical coupling condition, thus maximizing efficiency. Another reason to operate the wireless power transmission link in possibly approximated critical coupling condition is the reduced leakage field 1−k·Qsec. This reduces interferences. These interferences are radiated from the transmitter unit as a leakage field and are further increased by additionally generated interferences by the switching function of the resonant circuit rectification (non-linear distortions).
A major problem to be addressed in wireless power transfer concepts is the altering coupling (k) due to shifted geometrical properties of the coupling link and/or a changing secondary quality (Qsec) of the resonant circuit in the receiver unit. Qsec is itself a function of the load resistor RL, which in reality is not usually constant. Often, a stable output voltage or a stable output current is required and RL is determined by the power consumer's load (e.g. light, heat, audible power, exercise intensity, etc.). It is therefore desirable to develop a method which lets the wireless power transmission link operate optimally, possibly regardless of load resistor RL influences. This comprises controlling of Qsec independent of RL and further comprises optimizing Qsec to increase or control the efficiency and/or the range of the power transmission. Such a process should enable a simple, cost-effective, efficient and reliable operation.